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Centre Flue Wet-back Scotch Boiler

By Brian Greeves

This is my first attempt at a “wet-back”, I’ve always promised to have a go to see if it had advantages over previously made “dry-backs” and water tube boilers used to power my ST Double 10 in my steam tug.

Initial tests have proved very successful and the boiler will maintain its working pressure of 75psig at full throttle with no difficulty.


Outer Shell

180mm x 105mm dia drawn copper, fitted with 40mm turret housing steam take off to superheater, pressure gauge and pressure relief valve set @ 80psig.

Lagged with 2 x layers of synthetic lagging and 4mm Mahogany strip and brass banded.

Centre Flue and rear combustion chamber

The centre flue is 32mm copper fitted with 3 x Galloway tubes set at 30 degrees at the combustion  chamber end.

Rear combustion chamber is 60mm dia x 24mm deep. Both are flanged, riveted and silver soldered.

The top / middle section also houses the 8 x 10mm return tubes and 15mm superheater housing tube.

Superheater assembly

The superheater tube is constructed of thick walled 5mm brass tube the flue ends terminate in a 12mm copper block, which when fitted sit inside the rear combustion chamber. The one end fits to the steam turret (dome) and the other to the forward smoke box. Both the smoke box and superheater are removable for cleaning etc.

Forward and rear boiler ends

These have both been formed from the same gauge copper sheet as the main shell. The rear end contains the outlets for the water gauge, clack valve, blow-down valve and water level electronic sensor. The sensor is 3.2mm copper “L” shaped and set in PTFE insulation set within a brass housing.

Front end houses the 9 x return flue tubes, centre flue and the 4 x smoke box mounting brackets.

Smoke box

The tapered smoke box is constructed from silver soldered1.6mm mild steel, 40mm deep at the top. The stack is also 32mm brass tube fitted with a blast nozzle from the exhausted steam to aid firing.

Smoke box is fitted to the end plate with 4 x 4mm bolts and is removable for cleaning.


The unit is fired using standard butane / propane LPG mix, burner is of home design using ceramic block 30mm in diameter. This gives a series of individual “flamelets” approximately 30 – 40mm long.


Rear Combustion Chamber             Front inner assy ready for                                                                 outer shell

Smoke box and firing and             Service end, water inlet etc

         Steam outlet